This is easily my least favorite Rails model validation scenario. An association exists and at least one associated item must conform to a specific condition for the owning side of the relationship to be considered valid. In this case, to avoid orphaning organizations

organization.rb

class Organization < ApplicationRecord
  has_many :users
end

user.rb

class User < ApplicationRecord
  # Soft-deleteable with `discard!` https://github.com/jhawthorn/discard
  include Discard::Model
  
  belongs_to :agency

  enum :role, {
    manager: "manager",
    owner: "owner"
  }
  
  validate :doesnt_relinquish_final_ownership

  private

  def doesnt_relinquish_final_ownership
    # What would you do? Solution must:
    #
    #  * Not allow `discard!` to be called on the final role=owner user of an organization
    #  * Not allow the final role=owner on an organization to be changed to role=manager
    #  * Cover both persisted and new_record? users on the organization.users association
    #  * Not result in an N+1 on organization.users
  end
end